110th Anniversary: Evaluation of CO2-Based and CO2-Free Synthetic Fuel Systems Using a Net-Zero-CO2-Emission Framework

Abstract

This work analyzes quantitatively the energy and exergy efficiencies of storing intermittent renewable energy in chemical fuels. In the future energy system, chemical fuels provide a very effective approach for long-term storage and long-distance transport of renewable electricity. For the sake of completeness and simplicity, we consider both carbon-free fuels, namely, hydrogen and ammonia, and carbon-rich fuels, i.e., methane and methanol, synthesized using CO2 as the precursor. The latter are called CCU fuels as they constitute an application of CO2 capture and utilization (CCU), which is often advocated to be an effective approach toward climate change mitigation (though no consensus exists). Instead of focusing on the CO2 conversion step, we apply a system-oriented perspective, grounded in the net-zero-CO2-emission framework, to quantify merits and drawbacks. In such a framework, we consider eight systems and technology chains where, in the spirit of a circular economy, the only input is renewable electricity and the only output is a service, consisting in delivering either electricity to the grid on demand (power–fuel–power) or a fuel to propel a means of transportation (power–fuel–propulsion); no fossil carbon is used, and no net CO2 release to the atmosphere occurs. Providing the service of storing renewable electricity in chemical fuels obviously results in a loss of primary energy, which differs in the eight cases considered, depending on the chemical nature of the chemical fuel and on the number and efficiency of the individual steps to synthesize them. Power–CCU fuel–power systems exhibit an energy loss from 65% to 86%, whereas the energy loss of power–CCU fuel–propulsion systems increases to 83–94%. The energy loss of the corresponding systems using ammonia as fuel is similar, whereas that obtained when using hydrogen is significantly smaller, namely, 50–65% and 57–69% in the power–fuel–power and the power–fuel–propulsion case, respectively. Compared to hydrogen, the other energy carriers suffer from increased system complexity and consequently lower efficiency. Exergy analysis has shown low efficiency improvement potential for especially the fuel synthesis step, while the other steps in the chain (electrolysis, extraction from air of CO2 or nitrogen, fuel utilization, and associated compression) still exhibit higher improvement potentials.